[Clang][Driver] Valid -march value is not mandatory in AArch64 multilib

If a user passed an invalid value to `-march`, an assertion failure
happened in the AArch64 multilib logic.

But an invalid `-march` value is an expected case that should be handled
via error messages.

This patch removes the requirement that the `-march` value must be
valid.

Author: vhscampos

clang/lib/Driver/ToolChain.cpp

@@ -191,9 +191,10 @@ static void getAArch64MultilibFlags(const Driver &D,
   for (const auto &ArchInfo : AArch64::ArchInfos)
     if (FeatureSet.contains(ArchInfo->ArchFeature))
       ArchName = ArchInfo->Name;
-  assert(!ArchName.empty() && "at least one architecture should be found");
-  MArch.insert(MArch.begin(), ("-march=" + ArchName).str());
-  Result.push_back(llvm::join(MArch, "+"));
+  if (!ArchName.empty()) {
+    MArch.insert(MArch.begin(), ("-march=" + ArchName).str());
+    Result.push_back(llvm::join(MArch, "+"));
+  }
 
   const Arg *BranchProtectionArg =
       Args.getLastArgNoClaim(options::OPT_mbranch_protection_EQ);